1. Field of the Invention
The present invention relates to an optical pickup apparatus enabling to read two or more kinds of recording mediums having different reading wavelengths from each other such as a compatible optical pickup apparatus for DVD/CD, and particularly to an optical pickup apparatus using a semiconductor laser element formed of a one-chip laser diode for emitting two laser beams having different wavelengths from each other.
2. Description of the Related Art
Conventionally, a DVD/CD compatible reproducing apparatus sharing an optical pickup of a DVD reproducing apparatus with a CD reproducing apparatus has actively been proposed, and there are forms such as a DVD/CD compatible reproducing apparatus using an optical pickup of one-wavelength/bifocal type and a DVD/CD compatible reproducing apparatus using an optical pickup of two-wavelength/bifocal type.
Comparing a structure of a CD with that of a DVD, a thickness of a protective layer of the DVD (0.6 mm) is about one-half that of a protective layer of the CD. In case of reproducing both of optical disks using an optical pickup of unifocal type, when a light beam is gathered to achieve optimization to an information recording surface of the DVD, the protective layer of the CD through which the light beam passes is thicker than that of the DVD. Therefore, aberration such as spherical aberration occurs in the light beam and thus the light beam cannot be gathered optimally to an information recording surface of the CD. Also, since the CD is different from the DVD in a size of information pits formed for recording, it is necessary to form a beam spot having an optimum size to each of information pits on the information recording surface of the CD or the DVD in order to exactly read each of information pits.
A size of the beam spot is proportional to a ratio of a wavelength of the light beam to a numerical aperture of an objective lens for gathering the light beam on the information recording surface. That is, when it is assumed that a wavelength of the light beam is constant, the beam spot becomes smaller as the numerical aperture becomes larger. Thus, in case of reproducing the CD and the DVD by the optical pickup of unifocal type, when it is constructed so that a wavelength of the light beam is constant and the numerical aperture is adapted for, for example, the information pits of the DVD, the beam spot is too small with respect to the information pits of the CD and distortion occurs in a reproduction signal on reproducing the CD and an exact reading becomes difficult. Hence, a DVD/CD compatible reproducing apparatus using an optical pickup of bifocal type enabling to focalize on different positions on the same straight line and applying two laser beams for forming a beam spot with a proper size in correspondence with a size of each of information pits has become the mainstream.
For example, an optical pickup apparatus shown in FIG. 16 is a DVD/CD compatible reproducing apparatus mixing an optical path from a first light source 10 for CD and an optical path from a second light source 15 for DVD are mixed by a first beam splitter 13 acting as a prism to apply the mixed optical path to a bifocal lens comprising an objective lens and a diffraction element. A configuration and operation thereof will be described briefly.
In FIG. 16, according to a driving signal from a first driving circuit 11, the first light source 10 generates a laser beam (shown by a broken line) having a wavelength (780 nm) most suitable for an information reading from a CD, and this laser beam is applied to the first beam splitter 13 through a grating 12 for generating three beams. The first beam splitter 13 reflects the laser beam from the first light source 10 to guide the reflected light to a second beam splitter 14.
On the other hand, according to a driving signal from a second driving circuit 16, the second light source 15 placed at a 90xc2x0 angle with respect to the first light source 10 generates a laser beam (shown by a solid line) having a wavelength (650 nm) most suitable for an information reading from a DVD, and this laser beam is applied to the first beam splitter 13 through a grating 17 for generating three beams. The first beam splitter 13 transmits the laser beam from the second light source 15 to guide the laser beam to the second beam splitter 14.
The second beam splitter 14 guides the laser beam supplied through the first beam splitter 13, namely the laser beam from the first light source 10 or the second light source 15 to a bifocal lens 19 through a collimator lens 18. The bifocal lens 19 applies a information reading light gathered the laser beam from the second beam splitter 14 at one point to an information recording surface of an optical disk 21 rotated and driven by a spindle motor 20.
The laser beam from the first light source 10 (shown by a broken line) is gathered by the bifocal lens 19 to focus on an information recording surface C of the optical disk 21. Also, the laser beam from the second light source 15 (shown by a solid line) is gathered by the bifocal lens 19 to focus on an information recording surface D of the optical disk 21.
The reflected light, which is caused by applying the information reading light from the bifocal lens 19 to the optical disk 21, passes through the bifocal lens 19 and the collimator lens 18, is reflected by the second beam splitter 14, passes through a cylindrical lens 22 which is an astigmatism generation element, and is applied to a photodetector 23. The photodetector 23 generates an analog electrical signal having a level corresponding to the quantity of an applied light to supply the signal to an information data reproducing circuit 24 and a disk determination circuit 25 as a reading signal.
The information data reproducing circuit 24 generates a digital signal based on the obtained reading signal and further performs demodulation and error correction to the digital signal and reproduces an information data. The disk determination circuit 25 identifies a kind of the optical disk 21 based on a size of a beam spot formed at the time of applying a laser beam to the optical disk 21 to supply a disk identification signal to a controller 26 as disclosed in, for example, JP-A-10-255274 by the present applicant. According to the disk identification signal, the controller 26 drives and controls any one of the first driving circuit 11 and the second driving circuit 16 in order to selectively bring about a drive state.
The controller 26 drives only the first driving circuit 11 when the disk identification signal indicating the CD is obtained from the disk determination circuit 25. Accordingly, a laser beam emitted from the first light source 10 is applied to the optical disk 21 through an optical system comprising the grating 12, the first beam splitter 13, the second beam splitter 14, the collimator lens 18 and the bifocal lens 19. Then, the reflected light (return light) reflected by the information recording surface C of the optical disk 21 passes through the bifocal lens 19 and the collimator lens 18, is reflected by the second beam splitter 14, passes through the cylindrical lens 22, and is applied to the photodetector 23.
The controller 26 drives only the second driving circuit 16 when the disk identification signal indicating the DVD is obtained from the disk determination circuit 25. Accordingly, a laser beam emitted from the second light source 15 is applied to the optical disk 21 through an optical system comprising the grating 17, the first beam splitter 13, the second beam splitter 14, the collimator lens 18 and the bifocal lens 19. Then, the reflected light (return light) reflected by the information recording surface D of the optical disk 21 passes through the bifocal lens 19 and the collimator lens 18, is reflected by the second beam splitter 14, passes through the cylindrical lens 22, and is applied to the photodetector 23.
That is, the CD/DVD compatible reproducing apparatus comprises the first light source 10 for emitting the laser beam having the wavelength most suitable for the information reading from the optical disk 21 having a relatively low recording density such as the CD and the second light source 15 for emitting the laser beam having the wavelength most suitable for the information reading from the optical disk 21 having a high recording density such as the DVD to alternatively select the light source corresponding to the kind of the optical disk 21 targeted for reproduction.
As described above, while the DVD/CD compatible reproducing apparatus requiring two light sources requires the first beam splitter 13 serving as the prism and increases a cost compared with an optical pickup apparatus having one light source. When the first light source 10 is applied from one side of the first beam splitter 13, it is necessary to apply the second light source 15 from another side perpendicular to the first light source 10 and thus, there was a problem that space for placing the optical system increases and a size of the optical pickup apparatus becomes large.
The invention is implemented in view of the above-described problem, and an object of the invention is to provide an optical pickup apparatus in correspondence with two wavelengths enabling to miniaturize without using the prism.
In order to solve the problem, an optical pickup apparatus according to a first aspect of the invention, the optical pickup apparatus enabling to read information of a plurality of recording mediums having different reading wavelengths from each other, the optical pickup apparatus comprises:
a light emission unit including a first light emission source adapted to emit a first laser beam, and a second light emission source disposed adjacent to the first light emission source and adapted to emit a second laser beam whose wavelength is different from that of the first laser beam;
an objective lens fixed to a movable member, the objective lens for focusing the first and second laser beam;
a focus driving unit for driving the objective lens in at least one direction;
a photodetection unit; and
an optical system for guiding the first and second laser beams emitted from the light emission unit to the recording medium, the optical system for guiding a reflected light beam reflected by the recording medium to the photodetection unit, wherein the focus driving unit comprises a plurality of focus driving units;
the focus driving units are symmetrically placed with respect to the static balancing point of a support part for supporting the movable member;
each of the focus driving units has at least one pair of driving coils to which focus driving currents are supplied to generate a driving force in a focus direction;
the focus driving currents different from each other are supplied to the force driving units, respectively, to drive the objective lens in the focus direction with an inclination in relation to the focus direction.
In a second aspect of the invention, there is provided the optical pickup apparatus according to the first aspect of the invention, wherein any one of the first and second light emission sources is selectively driven to emit a laser beam.
In a third aspect of the invention, there is provided the optical pickup apparatus according to the first aspect of the invention, wherein the focus direction, in which the objective lens is driven, is a direction for focusing the first and second light beams at a predetermined position on the recording medium.
In a fourth aspect of the invention, there is provided the optical pickup apparatus according the first aspect of the invention, wherein the optical system including:
a grating for generating a pair of sub-beams from the first and second laser beams;
a half mirror for guiding the first and second laser beams to the recording medium while guiding a reflected light beam reflected by the record medium to the photodetection unit;
a collimator lens for converting the first and second laser beams into a parallel light beam; and a cylindrical lens.
In a fifth aspect of the invention, there is provided the optical pickup apparatus according the first aspect of the invention, wherein one of the first and second light emission sources is placed in a position which has an image height with respect to the objective lens;
the other of the first and second light emission sources is placed in a position which has not the image height;
when one of the first and second light emission sources is driven, the focus driving currents different from each other are supplied to the pair of the focus driving coils, respectively,
when the other of the first and second light emission sources is driven, the focus driving currents, which are equal to each other, are supplied to the pair of the focus driving coils, respectively.
In a sixth aspect of the invention, there is provided the optical pickup apparatus according the first aspect of the invention, further comprising an offset addition unit for adding an offset current to the focus driving current, wherein when one of the first and second light emission sources is driven, the focus driving current and the sum of the focus driving current and the offset current added by the offset addition unit are supplied to the pair of the focus driving coils, respectively.
In a seventh aspect of the invention, there is provided the optical pickup apparatus according the first aspect of the invention, wherein the focus driving current is generated based on a focus error signal.